Neck-down brass

Hi, im hoping that some of you out there can help me, the thing is that im having a 300 lapua magnum rifle made for me, I hava 338 lapua magnum brass that I need to make in to a caliber 30 and I dont know how in hell I do that or what I need to buy to make that, RCBS did tell me to just use the neck-die that is made for the 300LM, it looks to me like a to big jump to just neck it down in one step from 338 to 300. If anyone can teach me how to do this and tell me what to buy.
With thanks from Iceland,
Jakob

You should be able to run the 338 brass into your neck 300 die with no problem, Use good lube on the necks. I would however Anneal the brass after the neck down is done. They will work harden some with the neck down process.